Has applied white metal mortar round on subdued disk. A scarce piece of insignia- possibly unique.
Possibly unique, this WWI Trench Mortar collar disk (?) appears to be European made. Other than being made in the rectangular, the thumb screw is the same as found on issue collar disks. The trench mortar shell of a white metal, maybe silver and is applied to the disk. There is a small steading pin on the back to keep it from rotating; assuming that is was worn as the cloth insignia on sleeve, that being on the diagonal.
It is likely this was worn on an overseas cap, but not certain. Certain a scarcity for WWI disk collectors and scarcer still being trench mortar.
30mm at its widest.